Charles J. Shields

The photochemistry of 1,4-dihtdro-5h-tetrazole derivatives isolated in low-temperature matrices

Abstract Six 1,4-dihydro-5 H -tetrazole (tetrazoline) derivatives have been photolysed in Ar and N2 matrices at 12 K. The dimethyltetrazolinone ( 1a ) gave as the major product the same diaziridinone obtained previously from solution photolysis, but it also underwent a novel cleavage to MeNCO and, presumably, methyl azide. Iminotetrazolines ( 3a ) and ( 3b ) gave the corresponding carbodiimides ( 5a ) and ( 5b ), and iminodiaziridines ( 4a ) and ( 4b ); while the tetrazolinethiones ( 7a ) and ( 7b ) gave carbodiimides ( 8a ) and ( 8b ), respectively. Photolysis of the vinyl substituted tetrazolinone ( 9 ) proceeded differently in inert ambient-temperature solutions and low-temperature matrices. In the former an imidazolone ( 10 ) was the sole isolable product, while in the latter formation of diaziridinone ( 15 ) competed with an alternative cleavage giving t -butyl isocyanate and vinyl azide. These results are best interpreted on the basis of an intermediate biradical, which, however, could not be detected directly by matrix ir. spectroscopy.

Matrix electronic absorption spectra of the adducts of O2 with cyclopentadienylidene, tetrachlorocyclopentadienylidene, indenylidene and fluorenylidene

Abstract Four carbenes (cyclopentadienylidene, tetrachlorocyclopentadienylidene, indenylidene and fluorenylidene) have been generated by photolysis of the corresponding diazo precursors at 12 K in N 2 matrices containing 1–10% O 2 . In each case, an adduct of the carbene and O 2 was formed, having an electronic absorption with λ max in the range 395–460 nm. With cyclopentadienylidene, it was possible to show that the adduct with O 2 was probably the same species whose i.r. spectrum has recently been reported. On this basis and for additional reasons, all of the carbene-O 2 adducts are assigned carbonyl oxide structures.

The photolysis of 1-azido-4-methylbicyclo[2.2.2]octane and 1-azidoadamantane in low-temperature matrices

Abstract Photoysis of the bridgehead azides (1) and (5) in N2 and Ar matrices at 12K produces the strained bridgehead imines (2) and (6).

The photo-isomerization of cyclopentadienone O-oxide isolated in low temperature matrices

Discrepancies in recent reports from different laboratories on the i.r. spectrum of matrix-isolated cyclopentadienone O-oxide have now been reconciled by showing that two distinct isomers may be formed from cyclopentadienylidene and O2, and that one, a carbonyl oxide, may be converted photolytically into the other, a dioxirane.

The scope and limitations of cyanide groups as direction labels for the conformational analysis of matrix-isolated molecules of low symmetry: Part 1: Light-induced infrared dichroism of matrix-isolated 2- and 4-cyanophenyl azides

Abstract Photolysis of 2- and 4-cyanophenyl azides in solid N2 matrices at 12–20 K yielded anisotropic samples with linearly dichroic infrared spectra. An attempt was made to use the observed dichrosim, largely qualitatively, to correlate the directions of CN bonds with other bond directions in the molecules, thus developing a method of conformational analysis for matrix-isolated molecules. By this means, improved estimates for the directions of the transition moments of the azide stretches and the photo-active electronic absorptions were derived, and 2-cyanophenyl azide was shown, with a high degree of probability, to exist mainly as the anti conformer. Overlap of product and starting material v(CN) bands, however, proved to be a serious disadvantage of this approach.

The scope and limitations of cyanide groups as direction labels for the conformational analysis of matrix-isolated molecules of low symmetry: Part 2: 3-Cyanophenyl azide and two conformationally restrained derivatives

Abstract 3-Cyanophenyl azide and derivatives with ortho methyl groups were photolysed with plane-polarized light in N 2 matrices at 12–20 K. The resulting matrices containing the methyl-substituted derivatives had pronounced IR linear dichroism which was interpreted in a qualitative way to identify the predominant conformers of the matrix-isolated azides. The cyano groups functioned as direction labels , aiding the analysis of the results. Despite problems of band overlap with CN stretches, it was determined, to a high degree of probability, that phenyl azides with one ortho substituent exist in N 2 matrices predominantly as the anti conformer. In contrast, 3-cyanophenyl azide could not be photo-oriented in N 2 matrices to a significant or reproducible extent, which supports the expectation that the syn and anti conformers of this azide are virtually equal in energy and thus co-exist in comparable amounts.

Didehydroazepines from the photolysis of phenyl azide and 3- and 4-substituted phenyl azides isolated in low-temperature matrices

Phenyl azide, its 15N-labelled derivative (Ph15NNN), and a series of 3- and 4-substituted phenyl aiides have been photolysed in N2 and Ar matrices at 12 K. In all cases, the major decomposition pathways yield didehydroazepines with characteristic i.r. absorptions at ca. 1 890 cm–1. Neither the reaction pathway nor the characteristic group frequency of the didehydroazepines seem much affected by variation of the substituents.

An electron spin resonance study of the formation of radical cations in FSO3H/SO2 solution

Arenes which in FSO3H/SO2 solution show the n.m.r. spectra of the carbocations resulting from proton addition, also show the e.s.r. spectra of the radical cations resulting from electron loss. This provides a convenient technique for studying the e.s.r. spectra of radical cations in fluid solution down to ca. 180 K.A few compounds show special behaviour. 2,3-Dihydrophenylene and acenaphthene give rise to spectra which show that protons have been replaced, and which have a high g value. It is proposed that this results from the introduction of a disulphide bridge across the peri positions. Stilbene and diphenylacetylene show spectra with ‘normal’g values, but which are different from the spectra previously ascribed to the corresponding radical cations, and which appear to imply that there are non-equivalent ortho protons.

An electron spin resonance study of cyclopentadiene radical cations

If solutions of hexa-,penta-, tetra-, or tri-alkylcyclopentadienes in trifluoroacetic acid are irradiated with u.v. light, the e.s.r. spectra of the corresponding alkylcyclopentadiene radical cations can be observed. The radical cation Me5C5H+˙ can also be obtained from decamethylbi (cyclopentadienyl), Me5C5–C5Me5 under similar conditions. The spectra of Me6C5+˙, Me5C5H+˙, and Me4C5H2+˙ can be analysed in terms of single isomers. The sp3 carbon atom of the ring lies in the nodal plane of the SOMO, and substituents at this position show only small hyperfine coupling constants. The radical cation Me5C5H+˙ shows no evidence of fluxionality on the e.s.r. timescale up to 328 K. The spectra of Me4EtC5H+˙, Me3Et2C5H+˙, and Me3C5H3+˙ apparently relate to mixtures of prototropic isomers. Experiments with optical cut-off filters show that the primary photochemical process involves excitation of the protonated diene.